Comparative Study of the Thermal and MechanicalProperties of Nanocomposites Prepared by In SituPolymerization of e-Caprolactone and FunctionalizedCarbon Nanotubes

Vıctor H. Antolın-Ceron,1 Sergio Gomez-Salazar,1 Martin Rabelero,1 Vıctor Soto,2

Gabriel Luna-Barcenas,3 Issa Katime,4 Sergio M. Nuno-Donlucas11Departamento de Ingenierıa Quımica, Universidad de Guadalajara, Blvd. M. Garcıa Barragan # 1451,Guadalajara, Jal 44430, Mexico

2Departamento de Quımica, Universidad de Guadalajara, Blvd. M. Garcıa Barragan # 1451, Guadalajara,Jal 44430, Mexico

3Centro de Investigacion y de Estudios Avanzados del Instituto Politecnico Nacional, Unidad Queretaro,Libramiento Norponiente # 2000, Fracc. Real de Juriquilla, Queretaro, Qro 76230, Mexico

4Grupo de Nuevos Materiales y Espectroscopia Supramolecular, Departamento de Quımica Fısica, Facultad deCiencia y Tecnologıa (Campus Leioa), Universidad del Paıs Vasco (UPV/EHU), Bilbao 48940, Espana

As an effort to compare the influence of several typesof functionalized carbon nanotubes (CNTs) upon themechanical and thermal properties of nanocompositesprepared with a poly(e-caprolactone) (PCL) as matrixand functionalized CNTs as fillers; nanocomposites ofPCL–CNTs were studied in this study. CNTs were syn-thesized by chemical vapor deposition using dry etha-nol as the carbon source. High resolution scanningelectron microscopy, high resolution transmission elec-tron microscopy, and Raman and infrared spectroscop-ies were used to characterize the CNTs obtained. Fourchemical synthesis routes were exploited to add differ-ent types of chemical groups onto the surface of puri-fied CNTs. Specifically, the authors inserted: (i) N-meth-ylpyrrolidine, (ii) carboxyl and hydroxyl, (iii) urethane,and (iv) phenylmethanol groups onto CNTs surface.Nanocomposites were synthesized by in situ polymer-ization of e-caprolactone (e-CL) in presence of 1 wt%of each type of functionalized CNTs. Young’s moduli ofthe nanocomposites prepared with N-methylpyrrolidineor carboxyl and hydroxyl functionalized CNTs arehigher than the one of pure PCL, whereas all the me-chanical properties of the nanocomposites containingurethane or phenylmethanol groups evaluated at the

break point were higher than those of pure PCL.Thermal stability of all the nanocomposites studiedimproved with respect to pure PCL. POLYM. COMPOS.,33:562–572, 2012. ª 2012 Society of Plastics Engineers

INTRODUCTION

Carbon nanotubes (CNTs) sparked a research boom

due to their substantially improved mechanical, optical,

and electronic properties. For this reason, CNTs offer

promise to be used in different fields of nanoscience,

nanotechnology, bioengineering, and biotechnology [1].

One of these fields is the preparation of nanocomposite

materials. In this sense, the combination of CNTs with

polymeric macromolecules can be carried out by an easy

route and, there are high possibilities of obtaining new

nanocomposites with significantly improved properties

when compared with those of the parent matrix [2]. In

this regard, several polymers have been used as polymer

matrices to prepare new nanocomposites and include

thermoplastics [3], thermosetting resins [4], water-soluble

polymers [5], conjugated polymers [6], and electric con-

ducting polymers [7] among others.

The critical issue in the preparation of CNTs–polymer

nanocomposites is to achieve a high dispersion of CNTs

into a polymer-matrix. Since most of CNTs do not form

Correspondence to: S.M. Nuno-Donlucas; e-mail: gigio@cencar.udg.mx

Contract grant sponsor: Mexico’s National Council for Science and

Technology (CONACyT); contract grant number: CB-2008-101369.

DOI 10.1002/pc.22175

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2012 Society of Plastics Engineers

POLYMER COMPOSITES—-2012

aggregates, CNTs serve as effective reinforces of the ma-

trix. In this sense, a uniform dispersion of CNTs, induces

usually better thermal and mechanical properties than

those of pure matrix. Among the various methods known

to make this possible (solution mixing, melt mixing, and

in situ polymerization), the method of in situ polymeriza-

tion is more convenient because a higher percentage of

CNTs can be dispersed into the polymer-matrix [8]. The

method of in situ polymerization consists of dispersing

CNTs in a monomer followed by their polymerization. In

the initial step of polymerization, the dispersion of CNTs

into a liquid substrate seems to be an easy way for obtain-

ing a homogeneous distribution of CNTs. As the polymer-

ization progresses, the Brownian motion of CNTs is re-

stricted as a consequence of increased both molar mass

and viscosity of the new polymer-matrix created [9]. At

the end of the polymerization reaction, it is possible to

obtain a uniform dispersion of CNTs into a polymer-

matrix formed during polymerization. Several CNTs–poly-

mer nanocomposites have been prepared by this method,

for example: multi-walled carbon nanotubes (MWNTs)–

polyimide [10], single-walled carbon nanotubes (SWNTs)–

poly(methyl methacrylate) [11], MWNTs–poly(methyl

methacrylate) [12], MWNT–polypyrrole [13], and

MWNTs–polyurethane [14] among others.

To achieve an effective modification of CNTs with

polymers, two main methods have been developed: (i)

noncovalent attachment, i.e., polymer wrapping, and (ii)

covalent attachment, i.e., ‘‘grafting from’’ approach. Non-

covalent attachment consists of the adsorption of mole-

cules (e.g., surfactants or polymers) on the CNTs modify-

ing their surface energy, whereas covalent modification

considers the attachment of functional groups at the end

caps and sidewalls of the CNTs by chemical bonds.

CNTs–polymer nanocomposites with high grafting density

can be obtained through the ‘‘grafting from’’ approach

[8]. According to this route, a polymer is bound to reac-

tive CNTs by in situ polymerization of their precursor

monomer in the presence of previously functionalized

CNTs. Following this approach, macromolecules of poly

(acrylic acid) [15], poly(4-vinylpyridine) [16], poly(N-iso-propylacrylamide) [17], or poly(e-caprolactone) [18] havebeen successfully linked onto CNTs surfaces.

The insertion of specific chemical groups onto the

CNTs surface and its utilization to prepare nanocompo-

sites is of current interest. For the particular interest of

this study, the authors reviewed previous works about the

use of four chemical groups as: N-methylpyrrolidine, car-

boxyl and hydroxyl, urethane, and phenylmethanol

groups. For example, Maggini and Scorrano [19] reported

the insertion of N-methylpyrrolidine groups onto the sur-

face of fullerene C60 through a cycloaddition reaction.

Carboxyl and hydroxyl groups were added onto the sur-

face of CNTs by CNTs oxidation with oxygen, air, nitric

acid, concentrated sulfuric acid, acid mixture, and aque-

ous hydrogen peroxide. To achieve this addition, several

methods have been used as reported elsewhere [20, 21].

Chen et al. reported the preparation of MWNTs–polyur-

ethane composites using toluene 2,4-diisocyanate in their

synthesis, but they did not bonded isocyanate and ure-

thane groups onto the surface of MWNTs [22]. Buffa

et al., studied the insertion of phenylmethanol groups

onto the surface of CNTs reported elsewhere [23].

In this study, the authors compared the influence of

N-methylpyrrolidine, carboxyl and hydroxyl, urethane,

and phenylmethanol groups bonded onto the surface of

CNTs on the mechanical and thermal properties of nano-

composites prepared by in situ polymerization of e-capro-lactone. Poly(e-caprolactone) is a biocompatible polymer,

nontoxic to living organisms, and fully biodegradable.

One of the main reasons for selecting this biopolymer to

be used as a matrix of the nanocomposites prepared in

this study was to obtain data on materials with the poten-

tial to be used as scaffolds for tissue engineering. The

authors prepared CNTs by the technique of chemical

vapor deposition (CVD) using ethanol as a carbon source.

As-prepared CNTs were purified before any subsequent

treatment. Four different chemical paths were exploited to

achieve attaching of the above-mentioned chemical

groups onto the surface of CNTs. Each type of functional-

ized CNTs was used in preparation a type of PCL-based

nanocomposite. Thermal and mechanical properties of

these nanocomposites were evaluated.

EXPERIMENTAL SECTION

Fe(NO3)3�9H2O 98.2% and nitric acid 68% were pur-

chased from Golden Bell (Zapopan, Mexico). Alumina

boat, isopentyl nitrite anhydride 97%, e-caprolactone99%, 4-aminobenzyl alcohol 98% were acquired from

Alfa Aeser (Ward Hill, MA). Formaldehyde 38% and

chloroform 99.8% were purchased from Fermont, (Mon-

terrey, Mexico). Absolute ethanol was purchased from

Merck. N,N-dimethylformamide 99% was purchased from

Lancaster. Sarcosine 98%, octanol 99%, tin(II) 2-ethyl-

hexanoate (stannous octanoate) (SnOct2), potassium bro-

mide (KBr), FTIR grade, and toluene diisocyanate (TDI)

(80:20 w/w mixture of 2,4- and 2,6 isomers) were pur-

chased from Aldrich and used as received.

The CVD process was used to prepare the CNTs using

Fe as catalyst. This process is briefly described below. An

alumina boat was immersed in a ferric nitrate/ethanol

solution (5 wt%) for 24 h. After this time, ethanol was

evaporated at ca. 258C and the alumina boat was ther-

mally pretreated. To do this, the alumina boat was placed

at the center of a stainless steel tube (one in i.d. and 16

in long) and introduced into an electrical tubular furnace

(F2110 Barnstead-Thermolyne, Debuque, IA). The fur-

nace was heated at 4508C for 2 h to reduce Fe. Reduction

was considered complete by noticing a color change from

white to red of the alumina boat; at this point, the boat

was considered ready to be used. The growth of CNTs

was carried out in the alumina boat previously treated

containing Fe particles. Then the alumina boat was placed

DOI 10.1002/pc POLYMER COMPOSITES—-2012 563

again in the stainless tube. An ethanol–argon mixture was

prepared by bubbling argon (120 mL/min) through a 500-

mL Erlenmeyer flask containing 200 mL of ethanol and

kept at ca. 08C under local atmospheric pressure (640 mm

Hg). Then, the ethanol/argon mixture was introduced into

the stainless tube. The CVD process was carried out at

room temperature (ca. 258C) under local atmospheric

pressure for 6 h. During all CVD process, the furnace

temperature was maintained at 7208C. In a previous

study, the authors’ research group reported similar experi-

mental conditions to prepare CNTs [24].

The CNTs thus obtained were purified with steam at

6008C for 3 h using a quartz tube (0.5 in i.d. and 10 in

long) connected to a steam line.

Four different chemical routes were used to insert a

specific type of chemical group onto purified CNTs sur-

face, namely: (a) N-methylpyrrolidine, (b) carboxyl and

hydroxyl, (c) urethane, and (d) phenylmethanol groups.

Scheme 1 presents the chemical routes used to prepare

each type of functionalized CNTs.

For the insertion of the N-methylpyrrolidine groups,

authors followed the chemical route reported by Maggini

and Scorrano [19]. Briefly, 0.6 g of purified CNTs were

placed into a glass 100 mL batch reactor. Then, 2 g of

formaldehyde and 50 mL of N,N-dimethylformamide

were added to the reactor. This mixture was maintained

under agitation. On the other vessel, the authors prepared

a solution of N-methylglycine (sarcosine) in 20 mL of

N,N-dimethylformamide. Samples of this solution were

added periodically to the CNTs dispersion (5 g every 24

h). The reaction was carried out during 5 days at 1308C.The end product was separated by centrifugation and the

solid washed with dichloromethane and vacuum-dried in

an oven at 258C.

SCHEME 1. Chemical paths for the preparation of poly(e-caprolactone) based nanocomposites containing

(A) N-methylpyrrolidine-functionalized CNTs, (B) carboxyl-, hydroxyl-functionalized CNTs, (C) urethane-

functionalized CNTs, (D) phenylmethanol-functionalized CNTs.

564 POLYMER COMPOSITES—-2012 DOI 10.1002/pc

For the insertion of the carboxyl and hydroxyl groups,

0.5 g of CNTs were placed into the 100-mL reactor and

75 mL of nitric acid 7 M were added. The dispersion was

maintained under reflux in a Soxhlet for 6 h. The solid

was separated by centrifugation and washed with water

several times. The resulting purified solid was vacuum-

dried at 258C.For the inclusion of the urethane groups, the chemistry

of the polyurethanes was invoked. For this purpose, car-

boxyl–hydroxyl functionalized CNTs were dispersed in a

solution of 2 mmol of toluene 2,4-diisocyanate previously

dissolved in 30 mL of chloroform. The dispersion was de-

posited in the 100-mL reactor and maintained at 258Cduring 2 h under continuous stirring. After this, the solid

product was washed with chloroform and separated by

centrifugation. The residual solvent was eliminated in an

oven under vacuum at 258C.Finally, phenylmethanol groups were inserted onto the

surface of CNTS following the experimental method

reported by Buffa et al. [23] Briefly, 20 mg of CNTs

were sonicated for 30 min with o-dichlorobenzene. The

dispersion was placed into the 100-mL reactor along with

a solution made of 0.788 g (6.4 mmol) of 4-hydroxyme-

thylaniline in 12.3 mL of acetonitrile. The mixture was

stirred for 10 min under bubbling nitrogen. After this,

1.17 g (10 mmol) of isoamyl nitrate was added and the

new mixture was heated at 608C. This temperature was

maintained for 15 h under continuous stirring. The prod-

uct was filtered and washed with dimethylformamide in

excess and 2-propanol (twice). Then, it was dried at 258Cin an oven under vacuum.

Preparation of the nanocomposites was carried out by

in situ polymerization of the e-CL via ring-opening poly-

merization in the presence of 1 wt% of pure CNTs and 1

wt% of functionalized CNTs separately. The matrix of

nanocomposites PCL was synthesized in the 100-mL reac-

tor. Briefly, 10 g of e-caprolactone and 0.1 g of each type

of CNTs (purified or functionalized) were placed into the

reactor. Then, 0.02 g of tin(II) 2–ethylhexanoate and

0.033 g of octanol were added to this reacting mixture.

The mixture was stirred for 46 h at 1308C. A solid prod-

uct was obtained after cooling this mixture at room tem-

perature. This product was purified by recrystallization

with dichloromethane/petroleum ether and vacuum-dried

in an oven at 258C. Nanocomposites obtained were named

as indicated in Table 1, where the relationship between

the nanocomposite name and the type of functionalized

CNTs used in the synthesis, is showed.

CNTs were observed with high resolution scanning

electron microscope (HRSEM) model S-48000 of Hitachi.

This HRSEM has a Canyon of electrons by field emission

with a theoretical resolution of 1 nm. Samples were pre-

pared by placing them on a support of SEM followed by

gold coating.

CNTs were examined with a JEOL 2010 high resolu-

tion transmission electron microscope (HRTEM) operated

at 200 kV. Samples were prepared by mixing ca. 0.01 g

of CNTs with 3 mL of acetone at ca. 258C. The resulting

mixture was sonicated for 10 min. Then, using a Pasteur

pipette, an aliquot of the mixture was poured onto a Cu

grid. Solvent was evaporated by illuminating the Cu grid

with a light source using a 60-W solar lamp for 15 min.

Then, the sample was examined placing the Cu grid into

the HRTEM.

Vibrational behavior of the CNTs was monitored by

Raman spectroscopy. A Raman spectrometer model Lab

Raman II of Dilor equipped with a He–Ne laser was used

and operated at an excitation wavelength of 632.8 nm and

20 mW with an area spot of 2 lm using a 503 objective

and 2 cm21 error.

FTIR spectra of the CNTs were obtained with a Perkin

Elmer spectrophotometer model Spectrum One. For this,

pellets formed with KBr and perfectly dry samples were

prepared by compression at ca. 258C. Reported spectra

were taken using an average of 100 scans to reduce the

signal/noise ratio and a resolution of 2 cm21.

Differential scanning calorimetry (DSC) was used to

study the thermal properties of nanocomposites prepared.

The measurements were carried out in a TA Instruments

calorimeter model Q100 previously calibrated with in-

dium. All tests were performed under a nitrogen atmos-

phere. Calorimetric curves were recorded by heating the

samples from 280 to 1208C at 108C/min and the second

scan was reported. Sample weights ranged between 5 and

10 mg. The glass transition temperature (Tg) of the poly-

mer-matrix was evaluated by the inflexion point criteria.

Tensile stress–strain tests were recorded at ca. 258C to

measure some mechanical properties of the nanocompo-

sites studied. The tests were performed at a deformation

rate of 50 mm/min in a United machine model SFM-10.

The tested samples had a rectangular prism shape (55 314 3 3.5 mm 6 0.5 mm) according to the specifications

of the ASTM D882 rule.

Thermal gravimetry analysis (TGA) was carried out to

study the decomposition of the nanocomposites. Tests

were recorded in a thermobalance of Mettler–Toledo

model TGA/SDTA 851e. Sample masses were in the

range of 3–10 mg. TG curves were obtained by heating

samples from 20 to 500 at 108C/min under an argon

atmosphere with a flow rate of 75 mL/min.

RESULTS AND DISCUSSION

Figure 1A shows an HRSEM micrograph of purified

CNTs, where CNTs with several lengths (in some times

TABLE 1. PCL-based nanocomposites identification.

Nanocomposite name Fillers

Nanocomposite 1 N-methylpyrrolidine-functionalized CNTs

Nanocomposite 2 Carboxyl-, hydroxyl-functionalized CNTs

Nanocomposite 3 Urethane-functionalized CNTs

Nanocomposite 4 Phenylmethanol-functionalized CNTs

DOI 10.1002/pc POLYMER COMPOSITES—-2012 565

larger than 1 lm) can be observed. The authors carried

out purification of CNTs using steam. Tobias et al. [25]

demonstrated previously that pure steam at 1 atm pressure

is highly effective in purifying and opening CNTs. Mar-

uyama et al. [26] reported that when CVD process was

carried out using an alcohol as a source of carbon, the

CNTs obtained had a low content of impurities such as

amorphous carbon, metal particles, and carbon nanopar-

ticles due to the effect of OH radical attacking upon

carbon atoms. Our results corroborate this observation,

because the authors detected a loss of weight of ca. 24%,

from the unpurified CNTs, after 3 h of purification. This

weight loss is due to impurities eliminated with steam

and that accompanying our synthesized CNTs. In applica-

tions such as CNTs functionalization, the postsynthesis

treatments (e.g., purification process) have a crucial role,

because striking variations of some physical properties

(e.g., solubility in organic solvents and surfactant-based

solutions) have been reported between purified and

unpurified CNTs [27].

Figure 1B shows an HRTEM micrograph of the CNTs

synthesized in this study magnified at 10 nm. The inset

shows a highly magnified picture, where the multiple

walls that form part of these CNTs, can be observed. This

is a clear evidence of successful synthesis of MWNTs.

On the other hand, the morphologic characteristics of

MWNTs synthesized in this study and evaluated by meas-

urements using the software Image Pro Plus 6.0 of ca.

200 CNTs are: average length of 1.07 lm, although there

are MWNTs with a length of 9 lm; outer tube diameters

of the MWNTs ranged from 18 to 200 nm with an aver-

age value of 69 6 35 nm.

Figure 2 depicts the Raman spectra of purified and the

four kinds of functionalized CNTs prepared in this study,

whereas wavenumber of the Raman bands are listed in

Table 2. These are bands of the first order (at ca. 470 and

FIG. 1. (A) HRSEM micrograph of purified CNTs synthesized by

CVD. (B) HRTEM micrograph of pure CNTs synthesized by CVD.

FIG. 2. Raman spectra of (A) purified CNTs, (B) N-methylpyrrolidine-

functionalized CNTs, (C) carboxyl-, hydroxyl-functionalized CNTs, (D)

urethane-functionalized CNTs, and (E) phenylmethanol-functionalized

CNTs.

566 POLYMER COMPOSITES—-2012 DOI 10.1002/pc

1577 cm21) and second order (at ca. 1331 and 2654

cm21) Raman mode. The weak band at ca. 470 cm21 cor-

responds to the radial breathing mode (RBM) band, which

typically is detected in the range from 100 to 500 cm21

[28]. The RBM band is originated only by the presence

of SWNTs in CNTs sample. Therefore, it is evident that a

quantity not determined of SWNTs is mixed with the

MWNTs formed during the synthesis of the CNTs and

that was detected by HRTEM (Fig. 1B). There is an

inverse proportionality between the diameter of the

SWNTs (dt) and the RBM frequency (xRBM) as is

expressed by the equation:

xRBM ¼ A=dt þ B (1)

where A and B are constants. Taking advantage of the

values reported by Araujo et al. [29] for the A and B con-

stants (A ¼ 227 6 0.3 cm21; B ¼ 0.3 6 0.2 cm21)

authors calculated dt ¼ 0.48 nm of the SWNTs synthe-

sized in this study.

The D-band is associated to disordered sp2 carbon

materials. This band appears at ca. 1331 cm21 for purified

and functionalized CNTs. In addition, for functionalized

CNTs, a weak peak is resolved at ca. 1320 cm21 suggest-

ing some influence of the functionalization process on the

order of the walls of the functionalized CNTs.

The G-band was located at 1572 cm21 for purified

CNTs, while for functionalized CNTs, this band moves to

higher wavenumbers (see Table 2). This shift is more evi-

dent for phenylmethanol functionalized CNTs (see Table

2). This result is consistent with previous experimental

and theoretical works, where it was showed that doping

CNTs with either electron donors or acceptors produces a

noticeable shift to higher-frequencies for the G-band [30].

G-band is due to highly ordered carbon structures such as

the graphene sheets that form the walls of the CNTs. At

higher wavenumbers (1616 cm21) a shoulder of the G-

band can be seen (more clear in the spectra of all the

functionalized CNTs). This shoulder is the less well-

known disorder-induced band and is referred to as the

G*-band [31]. Table 2 also presents the ID/IG and IG*/IGratios obtained from the scattering intensities of the D

and G and G* and G bands. The intensity ratio of the D

mode to G mode can be used qualitatively to compare the

crystallinity of CNTs [32]. It is evident that the ID/IG and

IG*/IG ratios calculated from the four different types of

CNTs-functionalized, are higher than those calculated

from the purified CNTs. This result can be attributed to a

decrease in the structural order in the CNTs and a

decrease of the crystallinity of CNTs. Several authors

consider that an increase in disorder is caused by the for-

mation of the structural sp3 defects on the nanotube sur-

face, which is derived from functional chemical groups

bound to the walls of the CNTs [33, 34]. Therefore, this

result suggests strongly that external chemical groups are

bound to CNTs surface. The chemical nature of these

groups will be explained later. Finally, at higher wave-

numbers (ca. 2654 cm21) appears the G0 band, which is a

second order overtone of the D-band. It was reported, for

other CNTs samples, that the quality of a specific CNTs

sample depends of both G and G0 bands have similar

intensities [28]. As it can be seen from Fig. 2, the inten-

sities of G and G0 bands of each spectrum are similar.

Figure 3 shows the IR spectra of the purified and func-

tionalized CNTs. All the IR spectra were normalized to

unity taking as reference the more intense peak of each

spectrum, which appears at ca. 3450 cm21. Spectrum of

the purified CNTs is showed in Fig. 3 curve A. At 3450

cm21 appears an intense and broad band due to stretching

of a variety of hydroxyl groups present in different carbon

environments. The presence of hydroxyl groups in raw

TABLE 2. Wavenumber of the Raman bands and scattering intensities ratios (ID/IG and IG*/IG) of purified and functionalized CNTs.

Sample RBM band (cm21) G-band (cm21) D-band (cm21) G0-band (cm21) ID/IG IG*/IG

Purified CNTs 472 1572 1331 2648 0.72 0.02

N-methylpyrrolidine-functionalized CNTs- 472 1577 1331 2654 0.94 0.22

Carboxyl-, hydroxyl-functionalized CNTs 472 1579 1331 2658 1.46 0.49

Urethane-functionalized CNTs 472 1577 1332 2652 0.86 0.20

Phenylmethanol-functionalized CNTs 470 1582 1333 2661 1.04 0.23

FIG. 3. FTIR spectra of (A) purified CNTs, (B) N-methylpyrrolidine-

functionalized CNTs, (C) carboxyl-, hydroxyl-functionalized CNTs, (D)

urethane-functionalized CNTs, and (E) phenylmethanol-functionalized

CNTs.

DOI 10.1002/pc POLYMER COMPOSITES—-2012 567

SWNTs samples has been reported previously [35]. A

band at a lower frequency (1650 cm21) can be observed

and it is assigned to the stretching of carboxyl group,

while at 1110 cm21 other band was detected in the range

expected for stretching of C–O bond presented in ethers,

esters, alcohols, and phenols groups, which typically

appears in the range of 1240 to 1070 cm21. The authors

believe that the presence of C–O bonds as well as car-

boxyl and hydroxyl groups in the CNTs sample can be a

consequence of the use of alcohol as a carbon source in

the CVD process. The spectrum of the N-methylpyrroli-

dine-functionalized CNTs is presented in Fig. 3 curve B.

In this spectrum, the band assigned to carboxyl groups

moves to lower frequency with respect to the band

observed in the spectrum of purified CNTs (Fig. 3 curve

A), and appearing at 1630 cm21. On the other hand, a

weak band was detected at 1310 cm21. This band is origi-

nated by the stretching vibrations of the N–CH3 bond

[36]. Additionally, a medium intensity band appears at

lower frequency (627 cm21), which is assigned to rocking

vibrations of methylene groups. The last two bands sug-

gest that the methylene groups and the N–CH3 functional-

ity (both constituents of the heteroatomic ring of the

N-methylpyrrolidine groups) are bonded to the CNTs

walls. The shift of band due to stretching of the carboxyl

group to lower frequency can be considered as originated

by hydrogen bonds between the N atom and the carboxyl

groups both present in this functionalized CNTs. Spec-

trum of hydroxyl and carboxyl functionalized CNTs is

showed in Fig. 3 curve C. A double band at 1400 and

1385 cm21 appears and it is due to the stretching vibra-

tions of the OH and C–OH functionalities, respectively.

At 1641 cm21 was detected a band originated by the

stretching vibration of the carboxyl group. A weak band

at ca. 3200 cm21 appears as a shoulder of the intense

band resolved at 3448 cm21. This spectral contribution is

due to the existence of hydrogen bonds between the

hydroxyl groups inserted in the analyzed CNTs. Spectrum

of urethane functionalized CNTs is presented in Fig. 3

curve D. As it was observed from the spectrum presented

in Fig. 3 curve C, the spectrum of Fig. 3 curve D shows

also bands at 3448, 1642, and 1401 cm21 (the last as a

single peak). But now, at around 3280–3040 cm21,

clearly appears a shoulder of the band resolved at 3448

cm21. The authors consider this shoulder as due to

stretching vibrations of the N–H bond of urethane groups.

This statement is supported by two facts: (i) the absence

of a band due to asymmetric stretching vibrations of iso-

cyanate (N ¼¼ C ¼¼ O) groups typically resolved at 2264

cm21 and (ii) the fact that the reaction between hydroxyl

and isocyanate groups had been reported previously at

similar experimental conditions to the ones used by the

authors [36]. In this sense, our group has reported previ-

ously the synthesis of segmented copolymers via forma-

tion of urethane groups obtained from a chemical reaction

was carried out a temperature lower than the ones used in

this study [37, 38]. Additionally, the existence of urethane

groups in those copolymers was supported also by the

appearance of a band in the range of 3300–3000 cm21

[39]. Spectrum of phenylmethanol-functionalized CNTs is

showed in Fig. 3 curve E. The presence of aromatic rings

in the sample analyzed is supported for the following

spectral contributions: the bands resolved at 1591 and

1482 cm21 due to the stretching of the C ¼¼ C bond, the

band at 811 cm21 due to out-of-plane bending vibration

and the weak overtone detected at 1862 cm21. On the

other hand, the stretching vibration of the C–O bond pro-

duces the band detected at 1109 cm21, while bending

vibration of the O–H bond originates the band at 1287

cm21. These two bands are typical of the vibrations of

hydroxyl groups. Finally, asymmetric and symmetric

stretching vibrations of the methylene group produce

weak bands at 2921 and 2851 cm21, respectively. These

spectral contributions confirm the insertion of phenylme-

thanol group on these CNTs. Shao et al. [40] reported for

CNTs purified with steam that the chemical functionalities

detected by FTIR are produced by chemical groups effec-

tively attached to CNT walls. Therefore, since the spectra

above analyzed from samples previously purified, the

authors consider negligible the possibility that the spectral

contributions observed are due to chemical groups

attached to residual amorphous carbon.

Figure 4 shows the solubility of purified and function-

alized CNTs in chloroform. The photograph presented in

Fig. 4 was obtained after weighing 4 mg of each type of

CNTs and disperse in 8 mL of chloroform. The mixtures

were sonicated by 5 min and the photograph was taken.

A clear difference between the solubility of the purified

and functionalized CNTs is observed. For purified CNTs,

null solubility was observed, and CNTs was deposited at

the bottom of the container (container A) quickly. On the

contrary, a homogenous dispersion was observed for any

of the functionalized CNTs (containers B, C, D, E), which

is maintained stable for several days. This fact suggests

FIG. 4. Photograph of the CNTs dispersed in chloroform: (A) purified

CNTs, (B) carboxyl-, hydroxyl-functionalized CNTs, (C) urethane-func-

tionalized CNTs, (D) N-methylpyrrolidine-functionalized CNTs, and (E)

phenylmethanol-functionalized CNTs. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]

568 POLYMER COMPOSITES—-2012 DOI 10.1002/pc

the presence of chemical groups in the functionalized

CNTs surface with the ability to change the repulsion

between solvent molecules and the CNTs bundles, caus-

ing a reasonable good dispersion of functionalized CNTs

in chloroform.

Figure 5 depicts partial DSC curves of pure PCL and

their nanocomposites in the region from 20 to 908C. Tem-

perature interval, where the glass transition of PCL (a

subtle transition) appears, is not shown. PCL is a semi-

crystalline polymer and exhibits a typical endothermic

peak between 40 and 658C due to melting of its crystal-

line regions. The shape of the melting endothermic peak

of nanocomposites shows a significant difference with

respect to the one of pure PCL. Thereby, while in the

thermogram of pure PCL (curve A), only one melting

peak appears, the melting endothermic peak shows a

weak shoulder on the thermogram of the nanocomposites

[i.e., less clear in the thermogram of the Nanocomposite 2

(curve C)]. This difference can be induced by the higher

thermal conductivity of the CNTs when compared with

the organic matrix. Table 3 lists the glass transition (Tg),melting temperature (Tm), the melting enthalpy (DHm),

and the degree of crystallinity (Xc) of pure PCL as well

as of the four nanocomposites synthesized in this study.

The degree of crystallinity (Xc) was calculated by the

equation:

Xc ¼ DHm=DH0 (2)

where DH0 is the heat of fusion for completely crystal-

lized PCL and is taken as 136 J/g for the pure PCL [41].

Tg of the nanocomposites (with the exception to Nano-

composite 1) appears close to the one of pure PCL

(2648C). Tg of the Nanocomposite 1 is 2608C. In a simi-

lar way, melting temperature of all nanocomposites

appears almost at the same temperature of pure PCL

(578C). Melting enthalpy and the degree of crystallinity

of the nanocomposites is equal to pure PCL (Nanocompo-

site 1) or lower (Nanocomposites 2, 3, and 4). The high-

est decrease was observed for the Nanocomposite 4. The

described behavior indicates that the addition of CNTs

affected the melting process of the PCL. Similar results

were reported previously for other nanocomposites. Li

et al. [42] reported that the presence of MWNTs affected

the growth of crystals of polyamide 6 (PA6). Thus,

although MWNTs can act as heterogeneous nucleation

sites, they also retard the growth process of incipient

FIG. 5. DSC profiles of (A) pure PCL and PCL-based nanocomposites:

(B) Nanocomposite 1, (C) Nanocomposite 2, (D) Nanocomposite 3, and

(E) Nanocomposite 4.

TABLE 3. Glass transition temperatures, melting temperatures, melting enthalpies, and degrees of crystallinity (Xc) of PCL-based nanocomposites

containing 1 wt% of different types of CNTs.

Pure PCL Nanocomposite 1 Nanocomposite 2 Nanocomposite 3 Nanocomposite 4

Tg (8C) 264 260 265 265 264

Tm (8C) 57 56 57 58 58

DHm (J/g) 81 81 78 75 74

Xc (%) 60 60 57 55 54

FIG. 6. Tensile stress–strain curves of PCL and nanocomposites pre-

pared with PCL as matrix containing 1 wt% of fillers. Inset shows partial

tensile stress–strain curves of: (A) pure PCL, (B) Nanocomposite 1, (C)

Nanocomposite 2, (D) Nanocomposite 3, and (E) Nanocomposite 4.

DOI 10.1002/pc POLYMER COMPOSITES—-2012 569

crystals. This is caused by the reduction of the mobility

of PA6 macromolecules imposed by the presence of

rigid MWNTs. This produces a reduction in the degree

of crystallinity with respect to the pure polymer-matrix

as it was observed from the nanocomposites prepared by

authors.

Figure 6 shows tensile stress–strain curves of pure

PCL and their nanocomposites prepared in this study.

The inset included in Fig. 6 shows the region from 0 to

2% of strain. It is evident that, under unidirectional ten-

sile stress, the stress–strain curve of PCL and their

nanocomposites follows a behavior typical of hard and

brittle material [43]. In fact, for all nanocomposites,

both stress and strain increase without reaching a clear

yield point until reaching the deformation strain at

break. A higher stress is necessary to produce small

deformations for the Nanocomposites 1 and 2 (curves B

and C) when compared with pure PCL (curve A), while

for Nanocomposites 3 and 4 (curves D and E) less stress

is enough. Table 4 reports the Young’s moduli, ultimate

stress, deformation strain at break, and toughness of all

the materials tested. As expected, Young’s moduli of

Nanocomposites 1 and 2 are greater than the corre-

sponding of both pure PCL and the one of Nanocompo-

sites 3 and 4. On the contrary, the ultimate stress defor-

mation strain at break and toughness of the Nanocompo-

sites 1 and 2 are lower than the one of pure PCL. This

suggests that Nanocomposites 1 and 2 are harder and

more brittle than pure PCL. An opposite behavior was

detected for Nanocomposites 3 and 4. In fact, in these

cases, all the mechanical properties measured are higher

than the one of pure PCL (Table 4). An augment in the

magnitudes of mechanical properties of composites with

respect to the one of pure matrix indicates a positive

influence of the filler over the matrix, because the nec-

essary condition for improving the mechanical proper-

ties of any polymer composites (to achieve a well-load

transfer between CNTs and matrix) is met. Due to the

fact that all nanocomposites studied have equal content

of fillers, the results of mechanical tests obtained sug-

gest that urethane-functionalized CNTs and phenylme-

thanol-functionalized CNTs are better fillers than N-methylpyrrolidine-functionalized CNTs and carboxyl-

functionalized CNTs. Qian et al. [44] used other brittle

matrix: polystyrene (PS), to prepare nanocomposites

containing nonfunctionalized MWNTs (1 wt%) as fillers.

They found an increase of mechanical properties (i.e.,

the strength and elastic modulus) of their nanocompo-

sites with respect to the pure matrix due to a homogene-

ous distribution of MWNTs in the PS matrix. In this

respect, the degree of dispersion of the functionalized

CNTs into the PCL matrix of the nanocomposites stud-

ied by authors, could have been sufficiently homogene-

ous to induce the better mechanical performance

observed in Fig. 6 and Table 4.

Figure 7 depicts the TGA curves of pure PCL and their

nanocomposites. A partial range of temperatures from 250

to 4508C is showed in the inset. The inset clearly shows

that the loss of weight produced by the temperature rise

first started on the TGA curves of all the nanocomposites

(Fig. 7 curves B, C, D, E) and later on the TGA curve of

pure PCL (Fig. 7 curve A). This fact is due to the degra-

dation of the chemical groups attached to CNTs and

which are neither attached physically nor chemically to

PCL. At higher temperatures (between 378 and 3938C) achange in the thermal behavior of the nanocomposites is

observed. Thus, at temperatures higher than 3938C, it canbe observed that the loss weight of pure PCL (curve A),

occurs at temperatures lower than those of the nanocom-

posites (curves B, C, D, E). Since degradation of PCL

TABLE 4. Mechanical properties measured from tensile stress–strain tests of PCL-based nanocomposites containing 1 wt% of different types of

CNTs.

Pure PCL Nanocomposite 1 Nanocomposite 2 Nanocomposite 3 Nanocomposite 4

Young’s modulus (MPa) 207 6 14.8 232 6 14.2 232 6 16.0 175.5 6 14.9 126.7 6 9.0

Ultimate stress (MPa) 0.84 6 .06 0.57 6 0.05 0.45 6 0.01 3.3 6 0.3 6.2 6 0.6

Deformation strain at break (%) 0.98 6 0.1 0.4 6 0.1 0.8 6 0.1 1.1 6 0.1 3.5 6 0.3

Toughness(kPa) 4.3 6 0.3 1.7 6 0.1 3.2 6 0.3 4.1 6 0.6 116.8 6 8.2

FIG. 7. TGA thermograms of nanocomposites prepared with PCL as

matrix and 1 wt% of fillers. Inset depicts partial TG thermograms of:

(A) pure PCL, (B) Nanocomposite 1, (C) Nanocomposite 2, (D) Nano-

composite 3, and (E) Nanocomposite 4.

570 POLYMER COMPOSITES—-2012 DOI 10.1002/pc

chains occurs either for pure PCL and their nanocompo-

sites at the same range of temperatures, this result indi-

cates that the presence of functionalized CNTs confer a

better thermal stability to the polymer-matrix of the nano-

composites (PCL).

The type of modification of CNTs induced by the

PCL was dependent on the type of chemical groups

attached to the CNTs. In a previous study, Buffa et al.

[23] demonstrated that the polymerization of e-CL in

presence of hydroxyl functionalized CNTs produced

high amount of grafted PCL at similar conditions than

the ones used by authors. This fact and the results pre-

sented in the Figs. 5–7 suggest that in the cases of the

nanocomposites 2 and 4, hydroxyl-, carboxyl-functional-

ized CNTs and phenylmethanol-functionalized CNTs are

covalently attached to PCL. Scheme 2 shows the possi-

ble grafted polymer formed by these two nanocompo-

sites, whereas in the cases of nanocomposites 1 and 3,

the functionalized-CNTs have chemical groups that can-

not be chemically bound to e-CL during polymerization.

Consequently, a noncovalent attachment can only be

developed.

A comparison between the thermal and mechanical

behavior of the nanocomposites 4 and 2, reveals a better

performance for the Nanocomposite 4. On the other hand,

the thermal and mechanical response of the Nanocompo-

site 3 is better than the one observed in Nanocomposite 1.

Multiple factors influence this situation, e.g., the amount

of chemical groups effectively bounded between the ma-

trix and functionalized-CNTs (critical for nanocomposites

2 and 4), the creation of strong physical interactions (e.g.,

hydrogen bonding between the PCL and the functional-

ized-CNTs), as well as obtaining a uniform dispersion of

the CNTs into the PCL matrix, which are different for

each nanocomposite.

CONCLUSION

In this study, then authors prepared CNTs by CVD

technique and purified with steam. Purified CNTs were

chemically treated to graft four different types of chemi-

cal groups: (i) N-methylpyrrolidine, (ii) carboxyl and

hydroxyl, (iii) urethane, and (iv) phenylmethanol onto

their surfaces. Raman and Infrared spectroscopies demon-

strated that the above-mentioned chemical groups were

effectively grafted to purified CNTs. All functionalized

nanocomposites show markedly suspendability in chloro-

form. Functionalized-CNTs were used to prepare nano-

composites with PCL as the matrix by in situ polymeriza-

tion. Melting enthalpies and the degrees of crystallization

of all nanocomposites were lower than the one of pure

PCL. Depending on the type of functionalized-CNTs used

in the nanocomposites preparation, Young’s moduli, or

the mechanical properties at the break point of the studied

nanocomposites were better than those of pure PCL.

Covalent attachment was observed for nanocomposites 2

and 4, whereas nanocomposites 1 and 3 presented a non-

covalent attachment. Thermal stability of the nanocompo-

sites studied increases respect to pure PCL at higher tem-

peratures than 3938C.

ACKNOWLEDGMENTS

The authors are thankful to Mr. Francisco Rodriguez,

from CINVESTAV Queretaro, for his help in conducting

the Raman measurements.

SCHEME 2. Grafted PCL’s chains onto the surface of (A) carboxyl-, hydroxyl-functionalized CNTs and

(B) phenylmethanol-functionalized CNTs.

DOI 10.1002/pc POLYMER COMPOSITES—-2012 571

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